Finish the integration of webxr into the Cargo workspace (#35229)

- Run `cargo fmt` on `webxr` and `webxr-api`
- Fix clippy warnings in the existing `webxr` code
- Integrate the new crates into the workspace
- Expose `webxr` via the libservo API rather than requiring embedders to
  depend on it explicitly.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

ports/servoshell/egl/android/simpleservo.rs

@@ -35,7 +35,7 @@ pub struct InitOptions {
     pub coordinates: Coordinates,
     pub density: f32,
     #[cfg(feature = "webxr")]
-    pub xr_discovery: Option<webxr::Discovery>,
+    pub xr_discovery: Option<servo::webxr::Discovery>,
     pub surfman_integration: SurfmanIntegration,
 }